Refrigerated display cases which have a front opening for shopping convenience are well known in the art. As shown in FIG. 1, refrigerated display case 2 includes exterior member 3 and interior cabinet 4 between which outer conduit 3a and inner conduit 4a are formed. A refrigerating means, evaporator 5 and fan 6, is located within inner conduit 4a for refrigerating the air flow therethrough. Outlet ports 3b and 4b of outer and inner conduits 3a and 4a respectively are disposed on the upper side of the front opening and inlet ports 3c and 4c are disposed on the lower side of the front opening.
When refrigerated display case 2 starts to operate, the air flow through inlet port 4c is refrigerated by evaporator 5 and is continuously circulated through inner conduit 4a, outlet port 4b and inlet port 4c. Likewise, the normal temperature air flow through inlet port 3c is divided into two air flows at the bottom portion of the case 2 in outer conduit 3a. The two air flows merge at the upper portion of conduit 3a, and air is discharged through outlet port 3b. Accordingly, the refrigerated space in display case 2 is isolated from the ambient atmosphere by a double air curtain. Foods on shelves 7 in display case 1 are refrigerated and preserved in refrigerated condition by the double air curtain.
In this type of refrigerated display case 2, the inner curtain passage 4a and the refrigerating means must be frequently defrosted to remove any accumulated frost on the evaporator collected from the cooled air so that the frost does not imede the operation of the equipment. Such a defrosting operation in display case 2 is achieved by the use of electrical heater 8 located in front of evaporator 5 of the refrigerating means. Through defrosting by means of the electrical heater, the refrigerating mechanism is temporarily heated while allowing the air to continue to circulate through the two air curtains. Thus, the circulating air flowing through inner conduit 4a is warmed by heating of electrical heater 8. The warm air can then melt the frost built up on evaporator 5. At the same time, the exterior curtain is maintained. It is important to melt this frost as rapidly as possible, in order to minimize any rise in temperature of the refrigerated foods and to minimize collection of frost and the refrigerated foods from the higher humidity of the recirculated warm air.